It was probably the blood pumping through your pelvic area. I've gone in before for (what ended up being) cyst pain and the first couple of times I thought the same thing!! (That it was a heartbeat) but I've heard it every time. I did ask once what it was and that's what the ultrasound tech told me. If you were pregnant they'd definitely tell you. And show you the fetus on the monitir, of course!!